Sponge cloth production by the viscose process is well known (see for example DE C 1 193 237, whose United States equivalent is U.S. Pat. No. 3,068,545; DE A 26 56 968, whose United States equivalent is U.S. Pat. No. 4,232,128; and DE-A 27 07 308 whose United States equivalent is U.S. Pat. No. 4,198,968). First pulp, more particularly wood pulp, is converted with sodium hydroxide and carbon disulfide into an alkaline cellulose xanthate solution known as viscose. Glauber's salt (sodium sulfate decahydrate) is then added and uniformly mixed with the viscose. This sponge cloth raw material is then applied to an endless belt in the form of a layer to whichever height (thickness) is desired.
Cellulose regeneration takes place in a heated, alkaline coagulation bath. It can also be carried out in an acidic medium, for example dilute sulfuric acid.
Fiber-reinforced sponge cloth is generally produced by cotton noils being degreased with dilute NaOH and mercerized. Viscose, Glauber's salt and cotton fibers are then mixed together, generally by means of a kneader, to form a homogeneous mass. The fibers generally have an average length in the range from 0.5 to 4.5 cm. The sponge cloth material is then spread out and regenerated as described. After cellulose regeneration, the fibers have become bonded into the sponge cloth body. It is of particular importance that the distribution of the fibers is homogeneous and three-dimensional. To achieve this, the cotton noils have to be fully opened and wetted, or the fibers tend to curl and knot. Uniform internal reinforcement is then no longer ensured.
Glauber's salt has a very low melting point (about 32 to 33° C.). It therefore melts in the coagulation bath and is dissolved out. The disappearing salt crystals leave pores and voids behind. Finally, the sponge cloth is washed off to rid it of salt residues and adhering reaction products. After drying, it is cut into narrow lengths, which in turn are rolled up. The roll material can then be end-itemed into cloths of the desired size. If desired, they can additionally be printed beforehand.
Fiber-reinforced sponge cloths are brittle in the dry state, but do not break when subjected to mechanical loading. They are therefore marketed in dry form in few countries only. Moistened sponge cloths, by contrast, are more flexible and have a significantly better “hand”. To keep the sponge cloth moist, the sponge cloth lengths are impregnated with an aqueous solution of a hygroscopic salt, more particularly magnesium chloride. Excess liquid is removed by means of a pair of squeeze rolls. The moist cloths are then packaged, normally in a polymeric film impermeable to water vapor.
Sponge cloths incorporating a net are likewise well-known and are manufactured on a large scale. Production involves a sponge cloth material, which contains dissolved cellulose xanthate (viscose) and a pore former, more particularly Glauber's salt, as essential constituents, being applied through a die as a thin layer onto a carrier belt. The net or grid is placed on this web. Immediately thereafter a further layer of the sponge cloth material is applied thereon. The net or grid consists for example of polyester or else of cotton. This intermediate is then led through various coagulation, regeneration and wash baths in which the cellulose from the cellulose xanthate is regenerated. Coagulation and regeneration can take place in acidic or alkaline baths. Sponge cloths with a net are simpler to produce than fiber-reinforced sponge cloths, since a net does not tend to become knotty, unlike fibers.
Sponge cloths with a net are virtually exclusively offered for sale in pre-moistened form. To this end, they are treated with the abovementioned aqueous magnesium chloride solution and then enclosed in a water-impermeable film pack. Dry sponge cloths with net reinforcement break very easily on being subjected to mechanical loading. The net or grid is then the only thing which holds the sponge cloth together at the bend point.